This invention relates to a discharge lamp, particularly a high pressure automotive headlight discharge lamp with an arrangement for fixing an envelope of the lamp to a lamp base. The invention further relates to a method for manufacturing such a discharge lamp.
A wide variety of automotive high pressure discharge lamps are known in the art. These lamps normally comprise a relatively small discharge vessel surrounded by a larger envelope. The envelope is sealed around the discharge vessel, creating a monolithic discharge vessel-envelope assembly. The light of the lamp is generated by a discharge arc between two electrodes, producing a small light spot with very high luminous intensity. The light spot of the lamp is subject to strict positioning tolerances in order to keep the prescribed optical parameters of the headlight, into which the discharge lamp is installed. The light spot of the discharge vessel is positioned by adjusting the complete discharge vessel-envelope assembly into a proper position relative to a lamp base of the lamp and fixing the envelope (and thereby also the discharge vessel) in the adjusted position.
The envelope is held in the adjusted position relative to the lamp base using various solutions. In the method disclosed in U.S. Pat. No. 5,627,428 the envelope is held in place with the help of an outer end of the envelope, which extends beyond the sealing area between the discharge vessel and the envelope. This outer end of the envelope extends into a plastic holder portion of the lamp base, and the holder portion is melted around the outer end of the envelope. The melting of the holder portion is done with high frequency induced heating, and for this purpose, metal inserts are provided in the plastic holder portion. The holder portion itself is also melt sealed relative to the external shell of the lamp base. The metal inserts have no further role in the lamp, but only to facilitate the heating of the plastic parts. The problem with this fixing method is that the envelope inevitably develops stress around the indented portions at the sealing to the discharge vessel, and tends to break at these indented portions due to excessive vibration and similar mechanical loads.
Another method to fix the discharge vessel relative to the lamp base is disclosed in U.S. Pat. No. 5,378,958. In this known discharge lamp, the discharge vessel is held in place with the help of a metal clamp ring, which clamps around an end portion of the discharge vessel. The clamp ring is provided with metal legs or tongues, connecting to a fixation member, which latter is embedded in the lamp base.
A similar method is used in the D2 type discharge lamps manufactured by Philips Corporation, and described in Xenon HID Catalogue (Philips, June 2000). In this known lamp construction, the discharge vessel is enclosed by an envelope, similarly to the lamp described in U.S. Pat. No. 5,627,428, and the envelope is sealed to the discharge vessel near to its ends. A metal clamp ring surrounds the envelope between the indented sealing regions, but in the vicinity of the lower end of the envelope, so that the discharge chamber of the discharge vessel is not covered by the clamp ring. The clamp ring in this manner holds the discharge vessel-envelope assembly much closer to its centre of gravity, which ensures better mechanical support and resistance against deflections relative to the lamp base. On the other hand, the thermal load on the clamp ring is relatively high, being closer to the discharge chamber.
Though the above method also largely eliminates the cracking of the envelope at the indented sealing portions, other problems remain. The clamp ring is a closed ring to ensure sufficient clamping force around the envelope. However, in order to ensure a certain degree of flexibility of the metal clamp ring, the clamp ring must be provided with slight undulations in a circumferential direction. These undulations allow for natural differences between the diameters of the tubular envelopes due to usual manufacturing tolerances. The undulations also take up a part of the stress when the glass envelope expands under increased temperatures. However, these undulations also allow a certain degree of deflection of the envelope, particularly under shock and vibration during normal use of the lamp, for example in an automotive headlight. This deflection of the envelope may cause a degradation of the optical parameters of the headlight.
Therefore, there is a need for a discharge lamp structure that ensures a stable, substantially deflection-free fixation of the discharge vessel-envelope assembly relative to the lamp base, and which does not require expensive components and complicated manufacturing facilities, and which may be integrated into various types of existing production lines in a simple manner.